Digital signal processing is a broad field encompassing those operations which are done by and with digital logic circuitry and computers to condition or extract meaning from signals acquired from physical sensors or sources. Various specialized techniques using high performance circuits and programs are employed to enable signals to be processed numerically at high speeds. One class of digital signal processing circuits is digital filters. Digital filters are used for exactly the same purpose as analog filters--i.e., to pass signals in certain frequency bands and to attenuate signals in other frequency bands. Modern digital filtering is carried out by performing in time domain the successive multiplications and additions required to perform convolution, corresponding to multiplication in the frequency domain.
One well-known application of digital filtering techniques is "temporal averaging." Temporal averaging is an accepted method of reducing the noise in a picture or other waveform. It is accomplished by adding successive frames of an image or samples of a waveform. Weighted averaging often is employed, with one frequent approach being so-called exponential smoothing. Temporal averaging has heretofore been implemented using either discrete multiplier/accumulator integrated circuits together with appropriate analog-to-digital and digital-to-analog converters, or by using potentiometers. The latter approach requires manual adjustment of the potentiometers and introduces temperature dependency, potentiometer noise and other undesirable factors. The former approach requires the use of several integrated circuits, with attendant expenses. A good example of such a prior art multiplier/accumulator exponential smoothing system is shown in Analog Devices Data-Acquisition Databook 1984, Volume 1: Integrated Circuits, at page 12-12 (Analog Devices, Inc., Norwood, Mass. USA 1984).
It is therefore an object of the present invention to provide an improved adaptive digital filter suitable for use in temporal averaging.
Another object of the invention is to provide a digital temporal averaging circuit which uses fewer components than multiplier/accumulator designs.
It is a further object of the invention to provide a temporal averaging circuit based upon a standard voltage mode digital-to-analog converter.
Yet another object of the invention is to provide a less expensive digital filter for use in temporal averaging.